Dextrose may be produced in three crystalline forms, a hydrated form or .alpha. monohydrate form, and two anhydrous forms, i.e. the .alpha. anhydrous and .beta. anhydrous forms.
Solid dextrose is produced classically by crystallising supersaturated syrups with a high glucose content, and the crystals obtained are crystals of .alpha. monohydrate dextrose. Moreover, this process is described in the U.S. Pat. No. 3,039,935.
As for .alpha. anhydrous dextrose itself, it is obtained classically by dissolving crystals of .alpha. monohydrate dextrose in water, then performing crystallisation at temperatures in the range 60.degree. C. to 65.degree. C., under carefully controlled conditions of evaporative crystallisation under vacuum.
Furthermore, there are a number of processes for manufacturing anhydrous dextrose from starch hydrolysates, for example:
the process described in the U.S. Pat. No. 3,197,338, consisting in concentrating a starch hydrolysate to a dry matter content of dextrose of at least 95% on a dry basis, preferably at least 98% on a dry basis, crystallising this by a mixing process at a temperature in the range 75.degree. C. to 110.degree. C., and extruding it in the form of a ribbon in a zone which cools the product to a temperature of less than 65.5.degree. C., PA0 the process described in the U.S. Pat. No. 3,236,687, consisting in concentrating a starch hydrolysate to a dry matter content of dextrose of a value in the range 93% to 96% on a dry basis and subjecting it to a strong shear force in the presence of gas in order to form very small crystals of dextrose, PA0 the process described in the U.S. Pat. No. 4,059,460, consisting in preparing a concentrated melt of a glucose syrup with a concentration of 85% to 93% on a dry basis, at a temperature higher than 110.degree. C. The concentrated glucose syrup is then mixed by shearing force and cooled to a temperature of less than 95.degree. C. Finally, the glucose syrup is kept at a concentration of less then 93% and at a temperature higher than the temperature of crystallisation of .alpha. monohydrate dextrose, then shaped and converted into a solid mass. This solid mass is then granulated and dehydrated to a water content of less than 2%. PA0 that of directly using starch hydrolysates which contain, in addition to glucose, non-negligible proportions of other sugars with a higher degree of polymerisation (DP), for example DP2 (such as maltose) and DP3 (such as maltotriose). These sugars with a higher DP result in incomplete hydrolysis, whether it be chemical or enzymatic, of said starch hydrolysate. PA0 that of leading to mixtures of the two anhydrous forms of dextrose, at best in equivalent proportions, or even favouring the .beta. anhydrous form, and sometimes accompanied by .alpha. monohydrate dextrose, resulting in the incorporation of residual moisture as water of crystallisation. PA0 liquefaction of a starch milk using an .alpha.-amylase in such a way as to obtain a liquefied starch milk, PA0 saccharification of said liquefied starch milk using a glucogenic enzyme in such a manner as to obtain a crude saccharified hydrolysate, and PA0 optionally, microfiltration of said crude saccharified hydrolysate in such a manner as to recover a microfiltration permeate containing said crude starch hydrolysate and a microfiltration retentate. PA0 liquefaction of a starch milk using an .alpha.-amylase in such a way as to obtain a liquefied starch milk, PA0 saccharification of said liquefied starch milk using a glucogenic enzyme in such a manner as to obtain a crude saccharified starch hydrolysate with a maximum concentration of 80 wt. %, preferably a maximum concentration of 75 wt. %, and PA0 microfiltration of the crude saccharified hydrolysate in such a manner as to recover a microfiltration permeate containing said crude starch hydrolysate and a microfiltration residue. PA0 .alpha.-amylase: 20 to 2,000 KNU (Kilo Novo Units) per kilogram of dry substrate, temperature 80.degree. C. to 150.degree. C., duration of action 2 minutes to 15 minutes. PA0 amyloglucosidase: 4,000 to 400,000 international units per kilogram of dry substrate, temperature 50.degree. C. to 60.degree. C., duration of action 12 hours to a maximum of 24 hours, pH 4 to 6. PA0 pullulanase: 150 to 15,000 ABM units.
However, all these processes have two major disadvantages:
The crystalline dextroses obtained by these processes then have a strong tendency to agglomerate, which makes them difficult to handle. Furthermore, their flow characteristics are particularly poor.
In order to resolve the first and main disadvantage described above and to lead to the production of a dextrose with a more homogeneous crystalline structure, the patent FR 2.483.427 suggested concentrating a starch hydrolysate to a dry matter content of glucose of 92% to 99%, preferably about 95% to 99%, in a thin layer evaporator and at a temperature in the range 90.degree. C. to 135.degree. C.
However, the product obtained still contains more than 50% of the .beta. anhydrous form together with the .alpha. anhydrous form, and a non-negligible proportion of amorphous structure.
The anhydrous character is obtained in this process by using particularly high temperatures, but these operating conditions also have the direct consequence of increasing the proportion of the .beta. anhydrous form, which crystallises naturally at said temperatures.
As for the problem associated with contamination by sugars with a higher DP of the starch hydrolysates, two solutions have been suggested.
The first consists in optimising the process for preparing said starch hydrolysate.
However, although this solution reduces significantly the proportion of sugars with DP2 and DP3, it is especially difficult to obtain residual concentrations of less than 5%.
The second solution consists in using a nanofiltration process which eliminates all traces of these higher DPs, as is described in patent application FR 2.762.616, the owner of which is the assignee, or U.S. Pat. No. 5,869,297.
From all the preceding, however, it can be seen that there is an unfulfilled need to provide a crystalline .alpha. anhydrous dextrose of high purity.
In fact, all the processes of the prior art provide merely solid dextrose consisting of a mixture of the .alpha. and .beta. anhydrous forms, or even of the monohydrate forms, associated with relatively large amounts of DP2, DP3 or even higher DPs.